1. Field of the Invention
This invention relates to silicoaluminophosphate (SAPO) molecular sieves. In one aspect, this invention relates to small-pore SAPO molecular sieve membranes disposed on a porous support. In one aspect, this invention relates to a method for making and using SAPO molecular sieve membranes. In one aspect, this invention relates to a method of oxygen enrichment from air. In one aspect, this invention relates to the use of SAPO molecular sieve membranes for oxygen enrichment from air.
2. Description of Related Art
It will be appreciated by those skilled in the art that there are numerous industrial and other processes for which the use of oxygen having concentrations greater than the concentration of oxygen in air is highly beneficial. For example, the use of oxygen-enriched air in industrial combustion processes provides several advantages including fuel savings up to about 15%, CO2 reductions up to about 25% and efficiency increases up to about 20%. In fuel cells, the oxygen reduction rate is determined by the cathode catalyst and the oxygen concentration (mass transfer limitations). As a result, higher oxygen concentrations improve fuel cell performance. Empirically, at 400 mA/cm2 current density, if oxygen concentration increases by 10%, the fuel cell performance would increase by at least 10-20 mV depending on the pressure. The balance between the oxygen concentration and water releases from the catalyst surface, i.e. three phase area, is significantly improved as oxygen concentration increases.
The existing technologies for oxygen production include vacuum swing adsorption and cryogenic oxygen on site. However, as shown in FIG. 1, the costs associated with these technologies are very high, particularly when compared with the costs of membrane processes, which are less expensive, require less energy to operate, and do not require chemicals or regenerating absorbents to maintain. In addition, membranes are compact and can be retrofitted onto the combustion systems and fuel cells without complicated integration.
SAPO membranes, also known as SAPO molecular sieve membranes, are inorganic oxides largely composed of Si, Al, P, and O and can have a three-dimensional microporous crystal framework structure which provides cages, channels and cavities which enable separation of mixtures of molecules based on their effective sizes. SAPO crystals may be synthesized by hydrothermal crystallization from a reaction mixture containing reactive sources of silica, alumina, and phosphate, and an organic templating agent. See, for example, U.S. Pat. No. 7,316,727 to Falconer et al. which teaches SAPO membranes prepared by contacting at least one surface of a porous membrane support with an aged synthesis gel, forming a layer of SAPO crystals on at least one surface of the support as well as possibly in the pores of the support. The SAPO membranes produced in accordance with the Falconer et al. patent are said to have improved selectivity for mixtures of carbon dioxide and methane.
An important parameter in the use of SAPO membranes for gas separations is the separation selectivity of the membrane. For two gas components a and b, a separation selectivity Sa/b greater than one suggests that the membrane is selectively permeable to gas component a. Thus, if a gaseous stream containing both gas components a and b is provided to a feed side of the membrane, the permeate stream exiting the permeate side of the membrane will be enriched in gas component a and depleted in gas component b. Accordingly, the greater the separation selectivity, the greater the enrichment of the permeate stream in gas component a.
The kinetic diameter of a molecule is a reflection of the smallest effective dimension of a given molecule. It will be appreciated that a given molecule can have more than one dimension, which characterizes its size, if the molecule is not spherical. For example, O2 and N2 are diatomic molecules which are not spherical in shape but rather are cylindrical in shape. Thus, a “length” dimension of the cylindrical shape is a larger dimension than the smaller “waistline” diameter of the cylindrical shape. In transport phenomena, the molecule with the smallest effective “waistline” diameter is that which behaves as the smallest molecule, i.e., has the smallest kinetic diameter. For O2, the kinetic diameter is about 0.346 nm and for N2, the kinetic diameter is about 0.364 nm.